Method for bioleaching a metal present in a material

ABSTRACT

The invention relates to a method for recovering at least one metal present within a material, said material possibly including iron, said method including a step of supplying a ferrous ion, a step of supplying a ferric ion, and a step of bioleaching at least one metal present in the material by the ferric ions, each one of the steps being implemented by a particular bacterial population.

FIELD OF THE INVENTION

The invention relates to the field of biohydrometallurgy aimed at the recovery of ferrous and non-ferrous metals of interest present in a material by means of a bioleaching process. More particularly, the invention relates to an indirect bioleaching process for extracting at least one metal present in a material, and comprising the steps of supplying a ferrous ion in a first reaction space, of oxidizing the ferrous ion supplied to give a ferric ion in a second reaction space and of bioleaching said at least one metal via said ferric ion in a third reaction space.

PRIOR ART

Bioleaching processes consist in solubilizing metals present in the reduced or elemental state from various materials, such as ores, or solid by-products/waste resulting from the metallurgy industry, the mechanical industry, or the industry for converting or treating and recycling waste such as incinerator ash, batteries and spent catalysts, through the use of microorganisms. The metals thus solubilized are subsequently concentrated by physicochemical, hydrometallurgic or electrochemical techniques. An indirect bioleaching process comprises a step of generating a solution comprising ferrous ions, the ferrous ions then being oxidized to ferric ions by microorganisms. These ferric ions will leach the metals, i.e. spontaneously oxidize the metals present in the material and will result in the solubilization thereof when the oxidation-reduction and thermodynamic conditions are favorable to the reaction between the Fer(III) and the reduced or elemental metal under consideration. This solubilization by change of oxidation state is called redoxolysis and can be expressed for metals in the elemental state by the equation:

Me^(o)+Fe₂(SO₄)₃→MeSO₄+2FeSO₄

Me^(o) representing a metal of interest in an elemental form, and MeSO₄ representing the same metal of interest soluble in the oxidized state in a sulfate form. In the case of metal sulfides, the overall equation is expressed as follows: MS+O₂→M²⁺+SO₄ ²⁻, where M is a divalent metal which is solubilized from the mineral form by the action of microorganisms according to two routes identified in the literature: the thiosulfate route for metal sulfides which are insoluble in an acidic solution (for example: pyrite, molybdenite, tungstenite) and the polysulfide route for the oxidation of metal sulfides that are soluble in an acidic solution (for example, galena, sphlerite, chalcopyrite, arsenopyrite, chalcocite, etc.).

In its broad sense, the bioleaching process also comprises two supplementary mechanisms which result in the solubilization of the metals contained in the materials: acidolysis and complexolysis. Via acidolysis, the reduced metal is solubilized by protonation of the anion forming the initial compound with the reduced metal (metal oxides for example). This reaction can be expressed according to the following equation: MeO+H₂SO₄→MeSO₄+H₂O; the equation can be adjusted, mutatis mutandis, according to the various existing reduced forms of the metal in the material. The protons may be of chemical origin or may be generated biologically from elemental sulfur by oxidation to sulfuric acid. Via complexolysis, the metals are solubilized by the formation of a soluble complex with complexing agents, chelating agents or organic acids which are generated by certain species of microorganisms.

The indirect bioleaching processes are based on the use of populations of microorganisms which have the capacity to use a ferrous ion as electron source, oxidizing the ferrous ion to ferric ion accordingly. The application of this process to industrial waste allows the recovery after solubilization of metals of interest, either in the context of waste depollution, or in a process for recovering metals of interest for re-use thereof in new materials. Such processes are known. For example, in the publication by Bosecker (FEMS Microbiology Reviews, 20(1997) 591-604) and more recently in that by Lee (Waste Management, 32(2012) 3-18), several processes for bioleaching industrial waste are described. The pH can be controlled and regulated either by the presence of bacteria which specifically oxidize sulfur to sulfuric acid, or by adding sulfuric acid directly to the bioleaching medium. The industrial implementation of the bioleaching technique is generally carried out by spraying and recirculation of the leaching solution on piles or swathes of ores. In the case of waste in the form of granulates or powders, the usual bioreactors consist of two-phase or three-phase columns, that is to say structures in which the piled up or suspended waste is subjected to recirculation of the leaching solution which percolates on the solid matter or which immerses it (commonly defined by the term “packed bed bioreactor”). A gas phase provides for the oxygen and carbon dioxide needs (Pant et al., Chemical and biological extraction of metals present in E-waste: a hybrid technology. Waste Management 32 (2012); 979-990; Ilyas et al. Column bioleaching of metals from electronic scrap. Hydrometallurgy 101 (2010) 135-140). On a larger scale, “stirred tank” reactors (“stirred tank bioreactor bioleaching”) or “air lift reactors” are used to provide the best operating conditions.

Multiple-step processes exist in which the chemical processes of metal oxidation are physically separated from the biological processes of generation of the leaching agent (i.e. the ferric ion). Mention may, for example, be made of Brandl (Hydrometallurgy, Volume 59, (2001), Pages 319-326). However, the processes thus described do not make it possible to obtain a high degree of recovery of the metals of interest, or the processes are relatively slow (from two to a hundred days), thus decreasing the profitability of these processes. Thus, these processes are not satisfactory from the point of view of the yield and kinetics for recovering a not insignificant part of the metals of interest in order to make the process profitable from a technico-economic point of view.

Furthermore, some materials treated by bioleaching can comprise, depending on their nature, various compounds which can prove to be toxic and can reduce the microbial reliability and/or inhibit the growth and/or the activity of the microorganisms. Such compounds are, for example, heavy metals, arsenic(III) and arsenic(V), cobalt or organic molecules. These compounds reduce the efficiency or prevent the biological treatment of the materials comprising them.

Currently, the hydrometallurgical, electrochemical (such as electrorefining) and pyrometallurgical processes do not make it possible to recover metals with a sufficiently high yield when the material comprises a low amount of metals of interest and/or if it comprises metals which cannot be separated by the simplest techniques, such as: arsenic, antimony, mercury, bismuth, magnesium and thallium. These elements require pretreatments or the use of techniques which reduce the technical and economical profitability of the processes. Indeed, in this case, recourse is necessary to alkaline leaching processes with sulfides or acid leaching processes with chlorine, optionally followed by electrodeposition in a diaphragm cell, or by autoclave oxidation.

SUMMARY OF THE INVENTION

In order to at least partially solve the problems associated with the use of the current bioleaching processes, the inventors have developed a process for recovering at least one metal of interest present in a material, in which the bacterial populations used during the process are less exposed to the presence of compounds which are toxic and/or which inhibit the steps of a bioleaching process, in particular by using cultures of bacteria immobilized by embedding and through the effect of the phenomena of diffusional limitation of matter within the embedding matrix, which are inherent in this technique. Thus, even in the presence of such compounds, the bacterial populations used are able to allow the various steps of a bioleaching process to go ahead.

One objective of the invention is therefore to implement a process which allows the bioleaching of at least one metal of interest present in a material, the implementation of which makes it possible to recover the at least one metal of interest with a higher yield, while at the same time having a process that is faster. Another object of the invention is to implement a process which makes it possible to carry out a bioleaching of a wide panel of materials having variable compositions with regard to their qualitative compositions of non-ferrous metals, noble metals and “contaminants” such as arsenic, antimony, mercury and bismuth, magnesium, thallium, chlorine, fluorine, and polycyclic aromatic hydrocarbons (PAHs). The invention also allows a faster process through the use of embedded concentrated bacterial cultures, thereby ensuring a higher density of cells, and therefore of biocatalytic agent, than in free culture.

The invention is defined by the independent claims. The dependent claims define preferred embodiments of the invention.

According to the invention, there is provided an indirect bioleaching process for extracting at least one metal present in a material, and comprising the steps of:

-   -   a) supplying a ferrous ion in a first reaction space,     -   b) oxidizing the ferrous ion supplied to give a ferric ion in a         second reaction space,     -   c) bioleaching said at least one metal via said ferric ion in a         third reaction space,         characterized in that the step of supplying a ferrous ion is         carried out     -   i) either by bioleaching of an iron present within the material         by a first iron-oxidizing bacterial population (B1) which is         embedded in a first embedding matrix and immersed in a first         reaction medium present within the first reaction space,     -   ii) or by acid leaching of an iron present within the material         by adding a strong acid, preferentially sulfuric acid, to the         first reaction space,     -   iii) or by adding ferrous ions,         so as to obtain a ferrous ion concentration of between 2 g/l and         50 g/l in the first reaction medium,         and in that the steps of oxidizing the ferrous ion and of         bioleaching said at least one metal are each carried out in a         reaction space comprising a reaction medium and a bacterial         population (B2, B3) comprising at least one population of         iron-oxidizing acidophilic bacteria, each of the bacterial         populations (B2, B3) being embedded in a cell embedding matrix         immersed in each of the reaction media of each reaction space,         it being possible for the reaction media, the bacterial         populations and the materials present in the various reaction         spaces to be transferred between these various reaction spaces,         and in that an iterative oxidation of a ferrous ion, resulting         from the bioleaching of said at least one metal, to give a         ferric ion is carried out by the bacterial population (B3)         present in the third reaction space,         and in that the reaction spaces are different and the bacterial         populations (B1, B2, B3) are not identical.

Indeed, by virtue of the presence of a bacterial population embedded in an immersed cell matrix, it is possible to have a high cell density within the reaction spaces, leading to volumetric oxidation rates greater than the volumetric oxidation rates of the conventional processes in free cultures. Furthermore, by virtue of the presence of the bacteria within an embedding matrix, the presence of molecules within the material which are optionally released into the reaction medium and which have a harmful capacity potential toward microorganisms is not an obstacle to carrying out the process according to the invention, owing to the reduction in their harmful capacity by the presence of an embedding matrix and of the diffusional limitation phenomena that said matrix involves. With a cell embedding matrix, it is also possible to carry out mixed cultures of cells (or bacterial consortia), the same matrix comprising several layers of bacteria, and/or several bacterial species or genera, in proportions which can be established for example by virtue of various initial levels of inoculation within the embedding matrix depending on the nature of the material to be treated in order to bioleach at least one metal of interest. Finally, with such a process, the microorganisms are physically separated from the reaction medium and from the material, allowing easy recovery of the bioleached metals in solution in the reaction medium, of the embedded bacteria, allowing re-use thereof, and of the residual materials at the end of the process.

Another objective of the invention is to have a process in which the microorganisms used during the various steps are more adapted to unfavorable reaction conditions within the reaction media where the various steps of a bioleaching process take place. Each of the bacterial populations used can thus be specifically adapted to a predetermined step of the process according to the invention.

Thus, according to one preferred embodiment of the invention, the process comprises, in the first reaction space, a first bacterial population (B1) embedded in a first cell embedding matrix, said first bacterial population having, prior to its use in step a) of the process and when the iron is present within the material, undergone an acclimatization by culturing said first bacterial population in an acclimatization medium, said acclimatization medium comprising the material, said acclimatization medium being replaced with a fresh acclimatization medium when the redox potential of the acclimatization medium reaches a value greater than 550 mV, preferentially greater than 600 mV, and even more preferentially greater than 650 mV, said fresh acclimatization medium being supplemented at each replacement with an increasing concentration of material, said increasing concentration changing in steps of 2 g/l to 20 g/l of material, until the concentration of the material in the medium is between 2 g/l and 200 g/l, more preferentially between 50 g/l and 150 g/l, and even more preferentially the concentration is equal to 70 g/l and 80 g/l.

Indeed, the process comprising these steps is more effective since the microorganisms used are acclimatized before their use, and it is possible to separately acclimatize each of the bacterial populations according to the step in which it will be used, in particular by modulating the concentration of iron and/or of material present in the acclimatization media.

More preferably, the process according to the invention uses bacterial populations (B1, B2, B3) comprising acidophilic bacterial in pure cultures or in mixed cultures having at least one characteristic from each of the following groups selected from the groups comprising heterotrophic, mixotrophic, autotrophic, chemoautotrophic or chemolithoautotrophic bacteria which oxidize iron and/or sulfur and/or reduced forms of sulfur, psychrophilic, mesophilic, moderately thermophilic, hyperthermophilic and acidophilic bacteria, said bacteria having a growth and activity pH of between 0.5 and 3.0, and more preferentially the group comprising the bacteria Acidiferrobacter thiooxydans, Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, Leptospirillum ferriphilum, Acidimicrobium ferrooxidans, Sulfobacillus thermosulfidooxidans, Acidithiobacillus caldus, Acidianus brierley, Sulfobacillus acidophilus, Actinobacterium sp., Acidocaldus organivorans and Alicyclobacillus ferroplasma.

The process according to the invention operates even better by using these bacterial populations, and these bacterial populations have shown a potential for acclimatization and propagation within a cell embedding matrix which is entirely satisfactory in the context of the bioleaching process.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The indirect bioleaching process for extracting at least one metal present in a material comprises the steps of:

-   -   a) supplying a ferrous ion in a first reaction space,     -   b) oxidizing the ferrous ion supplied to give a ferric ion in a         second reaction space,     -   c) bioleaching said at least one metal via said ferric ion in a         third reaction space,         characterized in that the step of supplying a ferrous ion is         carried out,     -   i) either by bioleaching of an iron present within the material         by a first iron-oxidizing bacterial population (B1) which is         embedded in a first embedding matrix and immersed in a first         reaction medium present within the first reaction space,     -   ii) or by acid leaching of an iron present within the material         by adding a strong acid, preferentially sulfuric acid, to the         first reaction space,     -   iii) or by adding ferrous ions,         so as to obtain a ferrous ion concentration of between 2 g/l and         50 g/l in the first reaction medium,         and in that the steps of oxidizing the ferrous ion and of         bioleaching said at least one metal are each carried out in a         reaction space comprising a reaction medium and a bacterial         population (B2, B3) comprising at least one population of         iron-oxidizing acidophilic bacteria, each of the bacterial         populations (B2, B3) being embedded in a cell embedding matrix         immersed in each of the reaction media of each reaction space,         it being possible for the reaction media, the bacterial         populations and the materials present in the various reaction         spaces to be transferred between these various reaction spaces,         and in that an iterative oxidation of a ferrous ion, resulting         from the bioleaching of said at least one metal, to give a         ferric ion is carried out by the bacterial population (B3)         present in the third reaction space,         and in that the reaction spaces are different and the bacterial         populations (B1, B2, B3) are not identical.

An indirect bioleaching process is defined as being a process in which a metal is oxidized and solubilized by an oxidizing agent, a ferric ion resulting from the oxidation of the ferrous ion by the bacteria; the solubilization of the metals is chemically carried out by the biologically generated Fe(III) oxidizing agent, as opposed to other microbial mechanisms based on enzymatic activities which require contact between the cell membrane of the bacterium and the material (direct or “contact” bioleaching). The term “non-identical bacterial populations” should be understood to mean that the nature of the bacterial strains involved and/or the quantitative ratios of the various strains involved are not the same for the populations B1, B2 and B3.

The material can be in the form of industrial waste or industrial by-products comprising at least one metal of interest. The material can preferably be in the form of concentrated suspensions of fine particles (such as “sludge”), of scoria, of dust, of powders or of fine granulated material, or in any form which can be the subject of a particle size reduction in the form of powder or of granulated material by means of mechanical and/or physical processes. According to one preferential embodiment of the invention, the process applies to a material which is in powder form and which has a particle size of less than 1 mm, preferentially less than 0.5 mm, and even more preferentially less than 0.1 mm. The size of the particles of a material can, for example, be determined with the method described in standard ISO 4497-1983 (“Determination of particle size by dry sieving—metallic powders”). By way of example, the materials can originate from scrap from metallurgical factories, from chemical/petrochemical industries (catalysts), from treatment residues for the recovery of metals hydrometallurgically (cementation residues) or electrochemically (anode sludge), from scrap from pharmaceutical, glass, pyrometallurgy, refining and metal finishing (galvanoplasty) industries, from scrap from transformation factories, from household appliances at the end of their lifetime (dust, ground materials), from the recycling of electrical and electronic waste, from batteries, from ash resulting from the incineration of household waste, from waste and residues resulting from the activities of the metal and mechanical industry (automobile industry, such as powders resulting from polishing brake pads, the aeronautical industry, etc.). Preferentially, the steps of supplying a ferrous ion, of oxidizing and of bioleaching of the process according to the invention are carried out in reaction spaces subjected to mechanical, pneumatic or hydraulic stirring ensuring suspension of the materials, and always preferentially also at a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially about 0.030 s⁻¹ to 0.050 s⁻¹. In the case of leaching by chemical acidolysis, no provision of oxygen is required. An acidification of the reaction medium can be carried out. It can be performed by adding sulfuric acid of chemical or biological origin or by the presence, in said reaction medium or in a connected reactor which flows into the various reaction spaces, of strains of Acidithiobacillus thiooxidans and of elemental sulfur or other sources of reduced sulfur such as thiosulfate at a concentration of from 2 g/l to 20 g/l, which is biologically oxidized to sulfuric acid according to the equation 2S⁰+2H₂O+3O₂→2H₂SO₄. Preferentially, the material comprises iron and the step of supplying a ferrous ion is carried out at least partially, i) either by bioleaching of the iron present within the material by a first iron-oxidizing bacterial population (B1) which is embedded in a first embedding matrix and is immersed in a first reaction medium present within the first reaction space, ii) or by acid leaching of the iron present within the material by adding a strong acid, preferentially sulfuric acid, to the first reaction space.

Preferentially, the material is selected from the group of industrial waste, by-products or residues comprising non-ferrous metals and/or noble metals, said material being composed of at least one metal optionally in combination with iron, said metal being selected from the group comprising copper, zinc, nickel, tin, aluminum, gold, silver, platinum, rhodium, ruthenium, iridium, osmium, palladium, titanium, cobalt, vanadium, molybdenum, tungsten, beryllium, bismuth, cerium, cadmium, niobium, technetium, indium, gallium, germanium, lithium, selenium, tantalum, tellurium, arsenic, antimony, bismuth, lead, and mercury, or a combination of these metals.

The steps of the process can be carried out in a bioreactor. The various biological steps can be carried out in “continuous” mode or in “semi-continuous” mode. The two embodiments consist of the permanent presence of a biomass embedded in the embedding matrix immersed within each of the reaction spaces. An embedding is defined as an incorporation of microorganisms in the matrix of a more or less rigid, synthetic polymer (polyacrylamide for example) or natural polymer (protein such as gelatin or polysaccharide such as cellulose, agar-agar or alginates). Thus, the embedding matrix may be in various shapes, such as, for example, parallelepipeds, spheres (or “beads”), cylinders or any geometrical shape permitted by the various bacterial encapsulation techniques and allowing their use in a bioreactor. In the semi-continuous mode, the reaction media and the material are processed batchwise and iteratively, whereas the biomass remains in place and does not require lag and growth phases prior to each reaction batch which are characteristic of the “batch” mode with nonembedded microorganisms: in semi-continuous mode, a reaction medium comprising a bacterial population embedded in an embedding matrix and the material if its presence is required are put in place in a bioreactor until the desired activity is complete. The reaction medium, and the material if it is present, are then transferred into another reaction space where another activity is carried out. In continuous mode, the reaction media and the material are continuously added to and removed from each reaction space at flow rates (for the liquid medium) and at weight hourly loads (for the pulverulent solid materials) which are predetermined so as to ensure residence times in the bioreactors that provide optimal treatment of the material.

It is also possible to replace the bacterial population present within a reaction space following each step of the process, the reaction medium remaining present within the reaction space, and also the material if it is present. In this embodiment, the same reaction space is used to carry out the entire process of bioleaching the at least one metal of interest, but this reaction space comprises a reaction medium, a bacterial population, and a material if it is present, which are specific according to the step of the process, thus defining the various reaction spaces temporally and no longer physically.

The step of supplying a ferrous ion can be carried out by bioleaching of an iron present in the material. In this case, the material is present within the first reaction space. This step can be carried out in any type of reaction space, for instance a bioreactor which makes it possible to use a bacterial population embedded in an embedding matrix by encapsulation and immersed in the reaction medium, in continuous or “semi-continuous” mode. The bioleaching is carried out by virtue of the presence of the first bacterial population, by means of redoxolysis and acidolysis processes, which convert the various forms of iron present in the material into ferrous ion. Preferentially, the first reaction medium has a pH of less than 3.0. This mode of supplying a ferrous ion will be preferred when the material has an intrinsic iron content of between 5% and 80%, for instance brake pad dust. The step of supplying a ferrous ion can, alternatively or additionally, also be carried out by adding a ferrous ion directly to the first reaction medium. This addition can be carried out for example by adding ferrous ions originating from a solution. Depending on the material to be treated, the objective of this step is to obtain a ferrous ion concentration in the first reaction medium of between 2 g/l and 50 g/l in the first reaction medium, more preferentially between 2 g/l and 10 g/l.

The step of supplying a ferrous ion is stopped, or when the process is carried out in continuous mode the predetermined flow rate of the reaction medium and the weight hourly load of material are determined, when a transition between the step of supplying the ferrous ion and the step of oxidizing the ferrous ion to give ferric ion appears. This transition corresponds to the moment starting from which the curve of redox potential of the medium, which is established during the bioleaching of iron to iron(II) at a minimum initial value of 350±50 mV (between 300 mV and 400 mV) according to the operating conditions, shows a tendency to increase so as to culminate at the value of 600±50 mV (between 550 mV and 650 mV) which corresponds to the major presence under the operating conditions of iron in the oxidation state Fe(III). This transition can, for example, be determined by specific assaying of iron(II) and iron(III) using the common method with ortho-phenanthroline and with hydroxylamine for example: the transition is effective when the concentration of Fe(II) in the reaction medium is at a maximum and it marks a tendency to decrease to the benefit of that of iron(III). Thus, when the redox potential reaches a value of 350 mV, more particularly 400 mV, and even more preferentially 450 mV, the first reaction medium, and the material if it is present, are transferred into other reaction spaces, where the contact between the reaction medium and the first bacterial population embedded in a first embedding matrix, if it is present, is stopped by replacement of the first bacterial population with the second bacterial population embedded in a second embedding matrix after separation of the material from the medium, for example by decanting and phase separation.

Preferentially, the reaction conditions for carrying out the step of supplying a ferrous ion comprise a weight ratio of the weight of the first bacterial population embedded in a first cell embedding matrix to the total weight within the first reaction space (weight of the reaction medium and weight of the first bacterial population embedded in a first cell embedding matrix) of between 5% and 74%, preferably between 5% and 30%. Still preferentially, the aeration rate is between 0.1 vvm and 2.0 vvm (volume of air injected into the reaction medium per volume of reaction medium and per minute), the air being optionally enriched with from 0.1% to 10% CO₂. The diffusion of the air within the reactor can be carried out by an element which provides a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹. Such an element may be a toroidal diffuser, or a fine bubble diffuser made of ceramic or of elastomers of EPDM type. The stirring of the medium aimed at suspending the solid material and shear stresses providing the best oxygen transfer are provided by elements combining axial and radial dispersion (propeller turbine or mixer for example). Still preferentially, the reaction medium is a Lundgren-Silverman 9K medium (or medium of 9K type), and the reaction temperature is between 15° C. and 40° C. according to the composition of the bacterial population used. Even more preferentially, the pH of the first reaction medium is between 0.5 and 3.0, and even more preferentially between 1.5 and 2.0.

A medium of 9K type is a medium for the growth and activity of iron-oxidizing acidophilic bacteria as described in 1959 by Lundgren & Silverman (Silverman M P and Lundgren D G, 1959, Studies on the chemoautotrophic iron bacterium Ferrobacillus ferrooxidans. I. An improved medium and a harvesting procedure for securing high cell yields, J. Bacteriol., 77, 642-647) and which can comprise (NH₄)₂SO₄, MgSO₄.7H₂O, K₂HPO₄, KCl, CaCl₂.2H₂O, and/or Ca(NO₃)₂, and FeSO₄ in hydrated or non-hydrated form. The composition of this medium can vary in terms of compounds, concentrations, acidification and provision of trace amounts of minerals (Mn, Zn, Co, Mo, Cu, B, etc.). Numerous variants and adaptations can be found in the scientific literature (Mackintosh medium in Lewis et al. 2011, modified 9K medium in Long et al. 2004) according to the materials treated, to the microorganisms used and to the bioleaching conditions.

Preferentially, step a) of supplying ferrous ions is carried out with a ratio of the volume of the first bacterial population (B1) embedded in a first cell embedding matrix to the volume of the first reaction space of between 5% and 74%, when an iron is present in the material, the pH of said first reaction medium is between 0.5 and 3.0, and the concentration of material within the first reaction space is, when said material contains an iron, between 20 g/l and 200 g/l, preferentially between 50 g/l and 150 g/l and even more preferentially between 70 g/l and 80 g/l of material in the reaction medium.

The step of oxidizing the ferrous ion supplied so as to give ferric ion is carried out in a second reaction space. If the material is present in the first reaction medium, it is possible to separate the material from the medium before the introduction of the second bacterial population by common methods for separating a solid in suspension in a liquid, for example by decanting followed by phase separation, filtration, coagulation, flocculation, thickening and clarification, hydrocycloning or centrifugation. This step of oxidizing the iron(II) to give iron(III) prior to the bioleaching makes it possible to obtain a higher overall yield from the process by virtue of a reduction of the time in the subsequent step of bioleaching the at least one metal. The second space can either be a separate reaction space, or the same reaction space as the first reaction space, but in which the first bacterial population has been replaced with a second bacterial population and after separation of the material from the reaction medium by the common techniques of decanting followed by phase separation, filtration, and/or centrifugation. This step can be carried out in any type of reaction space, for instance a bioreactor which makes it possible to use a bacterial population embedded in an embedding matrix by encapsulation and immersed in the reaction medium, in continuous or “semi-continuous” mode. The oxidation is catalyzed by the second bacterial population. The oxidation step is accomplished and stopped, or when the process is carried out in continuous mode the predetermined flow rate of the reaction medium is determined, when all of the ferrous ion present is oxidized to ferric iron. For example, this parameter can be determined by assaying the ions (for example with ortho-phenanthroline) or by monitoring the redox potential of the second reaction medium: when the measurement of this parameter reaches a stable maximum at a value of about 600±50 mV, all of the ferrous ions initially present are in the form of ferric ion. This oxidation step can be stopped either by transferring the second reaction medium into another reaction space, or by stopping the contact between the second reaction medium and the second bacterial population embedded in a second embedding matrix, carried out by replacing the second bacterial population with the third bacterial population embedded in a third embedding matrix.

Preferentially, the reaction conditions for carrying out the step of oxidizing a ferrous iron to ferric iron are a ratio of the weight of the second bacterial population embedded in a second cell embedding matrix to the total weight within the second reaction space (weight of the reaction medium and weight of the second bacterial population embedded in a second cell embedding matrix) of between 5% and 74%, preferably between 5% and 30%. Still preferentially, the aeration rate is between 0.1 vvm and 2.0 vvm (volume of air injected into the reaction medium per volume of reaction medium and per minute), optionally enriched with 0.1% to 10% of CO₂. The diffusion of the air within the reactor can be carried out by an element which provides a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹. Such an element can be a toroidal diffuser, or a fine bubble diffuser made of ceramic or of elastomers of EPDM type. Still preferentially, the reaction medium is a Lundgren-Silverman 9K medium, and the reaction temperature is between 15° C. and 40° C. according to the composition of the bacterial population used. Even more preferentially, the pH of the second reaction medium is between 0.5 and 3.0, and even more preferentially between 1.5 and 2.0, and still more preferentially the pH is equal to 1.75.

Preferentially, step b) of oxidizing the ferrous ions to ferric ions is carried out with a ratio of the volume of the second bacterial population (B2) embedded in a second cell embedding matrix to the volume of the second reaction space of between 5% and 74%, said second reaction medium consisting of the first reaction medium after step a) of supplying a ferrous ion, and the concentration of ferrous ions is adjusted to between 2 g/l and 50 g/l, and the pH of the reaction medium is less than 3.0.

The step of bioleaching at least one metal of interest consists of the leaching of the metals present within the material which spontaneously enter into an oxidation-reduction reaction with the Fe(III)/Fe(II) pair under the operating conditions of the step. This step can be carried out in any type of reaction space, for instance a bioreactor which makes it possible to use a bacterial population embedded in an embedding matrix by encapsulation and immersed in the reaction medium, in continuous or “semi-continuous” mode. According to this embodiment, the bioleaching is carried out on the crude materials or materials resulting from a step of acidification and/or extraction of substances which inhibit a bioleaching bacterial activity and/or an iron-oxidizing activity, or on materials originating from the step of bioleaching the iron which is intrinsic for the waste which contain the same. The third reaction medium consists of the second reaction medium. The amount of material suspended in the third reaction medium depends on the nature of these elements and on their content of inhibiting and/or toxic compounds. During this step of bioleaching the metal of interest present in the material, the ferric ions are reduced to ferrous ions in the bioleaching process by reaction with the at least one metal of interest, and the ferrous ions thus produced are re-oxidized to ferric ions by the bacterial population present in the third reaction space. This oxidation is iterative, i.e. it takes place as long as ferrous ions are produced during the bioleaching of the metal of interest. The step of bioleaching at least one metal present in the material is accomplished and can therefore be stopped when all of the iron present in the reaction medium is in oxidized iron(III) form, which reflects the fact that all the at least one metal of interest available and mobilizable within the material has been the subject of bioleaching and that no elemental or reduced form of the at least one metal of interest can be oxidized any longer by the ferric ion.

Preferentially, the reaction conditions for carrying out the bioleaching step are a ratio of the weight of the third bacterial population embedded in a third cell embedding matrix to the total weight within the third reaction space (weight of the reaction medium and weight of the third bacterial population embedded in a third cell embedding matrix) of between 5% and 74%, preferably between 5% and 30%. Still preferentially, the aeration rate is between 0.1 vvm and 2.0 vvm (volume of air injected into the reaction medium per volume of reaction medium and per minute), optionally enriched with 0.1% to 10% of CO₂. The diffusion of the air within the reactor can be carried out by an element which ensures a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹. Such an element may be a toroidal diffuser, or a fine bubble diffuser made of ceramic or of elastomers of EPDM type. Still preferentially, the reaction medium is a 9K medium, and the reaction temperature is between 15° C. and 40° C. according to the composition of the bacterial population used. Even more preferentially, the pH of the second reaction medium is between 0.5 and 3.0, and even more preferentially between 1.5 and 2.0 and still more preferentially the pH is equal to 1.75.

Preferentially, step c) of bioleaching said at least one metal is carried out with a ratio of the volume of the third bacterial population (B3) embedded in a third cell embedding matrix to the volume of the third reaction space of between 5% and 74%, said third reaction medium consisting of the second reaction medium after step b) of oxidizing the ferrous iron to ferric iron, a pH of the reaction medium being between 0.5 and 3.0, and the concentration of material is between 20 and 200 g/l, preferentially between 50 and 150 g/l and even more preferentially between 70 g/l and 80 g/l of material in the reaction medium, said material consisting of the material after leaching of the iron by the first bacterial population (B1) when the material contains iron, and/or the material resulting from the step of acidification and/or extraction of substances inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity.

The expression “bacterial population embedded in a cell embedding matrix” should be understood to mean that a biomass comprising a bacterial population has been, prior to its use in the process, embedded within a matrix for embedding bacterial cells, said matrix allowing the cells to be in contact with the liquid reaction medium by diffusion of the medium through the matrix. The term “immersed matrix” should be understood to mean that the biomass embedded in the matrix is present in the liquid reaction medium.

Each of the steps of the process can be carried out in a bioreactor of aerated stirred tank bioreactor type, used in semi-continuous mode, or optionally in cascade (“continuous flow series of tanks”), or optionally continuously (“aerated CSTR, aerated continuous stirred tank reactor”) or in a “gas lift” or “air lift” reactor. The reactors that can be used in the context of this invention can also allow the physical separation of the bacterial population within an embedding matrix in order to prevent any mixing with the materials with a view to ensuring the subsequent separation thereof while ensuring the transfer of material (nutritive salts, Fe(III), Fe(II), O₂, etc.). The bacterial populations embedded in a matrix may, for example, be physically contained in separate compartments within the bioreactor but through which the reaction medium can diffuse so as to ensure the transfers of material at the molecular and elemental level, such as buckets, filtration candles, or bags made of synthetic fabric, or of mesh made of natural or synthetic materials (PP, PE, PTFE, polyamide, PVDF, etc.) resistant to conditions of acidity and having a mesh such that the colonized matrix is contained in order to prevent it from mixing with the materials in suspension in the reactor. However, it is possible to envision the method with direct suspension of the colonized matrices in a reaction space containing a reaction medium and the materials with no physical separation between the two solid phases, which are consequently separated on the basis of their difference in density, for example.

According to one preferential embodiment, the process is characterized in that said step of bioleaching said at least one metal is reiterated when the amount of said at least one metal in the material after a bioleaching step is greater than 20% of the initial amount of said at least one metal within the material, and more preferentially greater than 50%, and even more preferentially greater than 80%. The amount of bioleached metal is defined according to the formula (1−(Mr/Mi))×100 where Mr is the content of metal of interest expressed in g/kg in the residual material (i.e. the material having undergone the bioleaching step(s)) and Mi is the content of metal of interest expressed in g/kg in the initial material, i.e. the material not yet having undergone a bioleaching step.

According to this embodiment, when all of the iron present in the third reaction medium is oxidized to ferric iron, indicating that the at least one metal of interest is no longer available within the material for bioleaching, the material is re-used in at least one bioleaching step, each supplementary step being carried out with a “fresh” third reaction medium, i.e. a medium resulting from the step of oxidizing a ferrous iron to ferric iron. This step can allow the bioleaching of a proportion of the metal of interest that is less available or less “mobilizable” within the material, thus increasing the total amount of metal of interest recovered.

According to one preferential embodiment, the process is characterized in that it also comprises at least one of the following steps:

extraction of at least one substance present in said material and inhibiting a bioleaching bacterial activity and/or an iron-oxidizing bacterial activity, said step being carried out before the step of supplying a ferrous ion,

acidification of the material by supplying an acid solution such that the material is in a solution of which the pH is stabilized between 0.5 and 3.0.

Although the bacterial populations used in the process are protected against unfavorable reaction conditions by their presence within an embedding matrix, it is possible to even further improve the yield from the process by separating, and/or by neutralizing, inhibiting or toxic components and/or by modifying the physicochemical properties of the material by acidifying it.

The at least one substance present in said material and inhibiting a bioleaching bacterial activity and/or an iron-oxidizing bacterial activity can, for example, be organic matters reputed to be toxic to autotrophic acidophilic bacteria, in particular to Leptospirilum ferrooxidans, for example flame-retardant compounds, organic molecules such as HAP, PCB, PCDD or PCDF but also mineral molecules or elements which exercise a harmful role against microorganism viability and/or activity, for instance magnesium, cobalt and arsenic in various oxidation states. The techniques which make it possible to extract at least one inhibitor compound can be of various natures, for instance physical (densimetric separation, flotation, flocculation, etc.), chemical (by complexation reactions, oxidation-reduction reactions, by reaction of acid (acidolysis), organic solvents, surfactants) or else electrochemical. By way of example, mention may be made of magnesium that can be found in specific pulverulent waste or by-products resulting, for example, from the automobile industry (brake pad dust, clutch dust, etc.). Its bacteriostatic or even bactericidal characteristics, depending on the concentration, make their removal thereof preferable for good development of the biological process. A method of removing magnesium from brake pad dust is an acidolysis reaction carried out by suspending the material in a solution of sulfuric acid at a pH of between 1.0 and 2.0.

In the case of materials which have a pH value greater than those that are preferred in the subsequent steps of the treatment (between 0.5 and 3), this step also makes it possible, concomitantly or after separation/neutralization of the inhibiting and/or toxic compounds, to adapt the pH of the materials by acidification. This step is generally carried out by suspending the materials in an acid solution (sulfuric acid for example) or in culture medium (Lundgren-Silverman 9K type), with stirring and addition of acid (sulfuric acid for example), and control and monitoring of the pH for adjustment to a value of between 0.5 and 3.0. The regulation of the pH of the materials is considered to be complete when the target value is reached and remains stable.

According to one preferential embodiment of the invention, the process comprises a step of embedding a bacterial population (B1, B2, B3) in a cell embedding matrix, and the embedding step comprises the steps of:

supplying a liquid embedding medium,

supplying a cell embedding matrix compound in the embedding medium according to a weight-to-volume ratio between the embedding matrix compound and the embedding medium of between 5 g/l and 400 g/l,

supplying an inoculum of the bacterial population in the embedding medium,

forming a cell embedding matrix comprising the bacterial population.

A cell matrix compound may comprise components forming a matrix compound, such as monomers, polymers, prepolymers, protomers, ligand agents, or polymerization promoters or initiators.

The presence of an embedding matrix encapsulating the bacterial population makes it possible to at least partially protect the bacterial population against inhibiting and/or toxic compounds present in a reaction medium. According to this preferential embodiment, it is possible to encapsulate a bacterial population so as to make it possible to obtain a higher yield from the process according to the invention. The presence of an embedding matrix also makes it possible to carry out the process using material contents (or “pulp densities”) higher than the contents used in conventional processes. Furthermore, the presence of a bacterial population within an embedding matrix allows the use of a high volumetric concentration of bacteria (high cell density). Finally, the embedding of the bacterial populations within an embedding matrix makes it possible to increase the yield of recovery of the metal of interest, by carrying out the process in semi-continuous mode, or even in continuous mode, of the bacterial populations, contrary to the process conventionally carried out in batch mode using bacteria in free culture.

The embedding matrix is a solution of matrix compounds (monomers, prepolymers, protomers or polymers) having the capacity to encompass a bacterial biomass following the action of a chemical agent or of physical or photochemical conditions for gelling, crosslinking or polymerization (ligand agents, polymerization initiators or promoters, ultraviolet, condensation, etc.). Preferentially, the matrix compound is selected from the group of water-soluble and natural or synthetic polymers which can form hydrogels by ionotropic, photochemical or thermal gelling or by crosslinking or polymerization, said group comprising alginate, agar, gerlite (anionic heteropolysaccharide), chitosan, kappa-carrageenan, polyacrylamide, polyacrylamide-hydrazide, co-poly(N-isopropylacrylamide/acrylamide), polyethylene glycol, methacrylates based on monomers of methylacrylamide, hydroxy ethyl methacrylate or methyl methacrylate, epoxy resins, photo-crosslinkable resins, diethylene glycol ester, polyvinylpyrrolidone, silicone, polyvinyl alcohol, polyethylene glycol monomethacrylates or dimethacrylates or diacrylates, polyurethane hydrogel, hydroxyethyl methacrylate, photo-crosslinkable resins or a mixture of several of these compounds.

An embedding medium is a medium which makes it possible to mix a bacterial inoculum with a solution of matrix compounds for the purpose of obtaining a bacterial population embedded in a solid embedding matrix forming a hydrogel. This medium may for example be an aqueous solution at the pH adapted to the type of hydrogel used by means of an acidic or alkaline compound or a bacterial culture medium, for example of Lundgren-Silverman 9K type optionally devoid of forms of iron according to the compatibility of this element with the process for forming the hydrogel under consideration or any variant of this medium comprising only one or more elements of this medium.

The bacterial inoculum is a mass of bacteria, optionally separated from its culture medium. The bacteria are recovered beforehand from a bacterial cell culture. Preferentially, the bacterial inoculum will comprise a bacterial population of approximately 10⁸ to 10¹² bacteria/ml. Still preferentially and in particular when the iron can interfere with the process for forming the embedding matrix, at least one step of rinsing the bacterial population is present before the matrix-forming step, in order to remove the iron present in the culture medium of the bacterial population, ideally in a medium of Lundgren-Silverman 9K type devoid of iron forms at pH 1.75.

The formation of the matrix consists in bringing the bacterial inoculum into contact with a matrix compound in the presence of one or more (chemical, physical or photochemical) agent(s) enabling the gelling/crosslinking/polymerization of the matrix compound. The conditions according to the type of matrix compound used are well known to those skilled in the art, and can be found in the scientific and technical literature (for example in the publication by Nedovic V. and Willaert R., Application of Cell Immobilisation Biotechnolgy 2005 series “Focus on Biotechnology”, published by Springer).

By way of example, a bacterial inoculum resuspended in embedding medium after centrifugation and comprising about 10¹⁰ cells/ml is supplied. An embedding matrix is prepared using PVA (polyvinyl alcohol) and sodium alginate in combination at concentrations of between 1% and 40% and 0.5% and 20%. A semi-synthetic matrix is formed at respective final concentrations of 9% and 1% in order to encapsulate the bacterial populations. The matrix can be dissolved by heating at a minimum temperature of greater than 80° C. or sterilized (for example by heating the matrix at a temperature of greater than 121° C. for at least 20 minutes), then the matrix is mixed with the bacterial inoculum. A crosslinking, gelling or polymerization agent is then used depending on the nature of the matrix-forming compound and the mixture is then extruded by gravitation, mechanical, electromagnetic, ultrasonic or pneumatic shear, or any technique which allows the formation of structures encapsulating the bacterial populations, such as spheres or sheets for example. The polymerization agent may for example be, in the case of an alginate/PVA matrix, a solution comprising 3% to 6% (w/w) of H₃BO₃ (boric acid) and 0.5% to 2% of CaCl₂.2H₂O in which the encapsulating solution comprising the bacteria is extruded dropwise so as to form hydrogel spheres+/−3 mm in diameter. In the case of polyvinyl alcohol, a stabilization phase with phosphates or sulfates, as recommended by Zain (Process Biochemistry 46 (2011), 2122-2129) may be present. The crosslinking, in the case of polyvinyl alcohol, can alternatively be carried out by several cycles of freeze-thawing (technique known in the literature as freeze-thawing technique) and can provide sheets or thin layers of hydrogel which can subsequently be cut into cubes of desired size (3 mm-sided, for example). In another example using a matrix compound of polyethylene glycol dimethacrylate-based polymer type, a bacterial inoculum resuspended in embedding medium after centrifugation of the preculture and comprising about 10¹⁰ cells/ml is supplied. A synthetic embedding matrix is prepared by mixing the prepolymer of polyethylene glycol dimethacrylate having a molecular weight of between 200 and 2000 g/mol, in an aqueous solution at a final concentration of 100 to 200 g/l in combination with 5 to 10 g/l of polymerization promoter agent (N,N,N′,N′-tetramethylenediamine) and with a bridging agent at a final concentration of 5 to 10 g/l (N,N′-methylenebisacrylamide or triacrylformal for example). The polymerization of the aqueous solution of matrix compound also containing the bacterial inoculum is initiated with potassium persulfate (2.5 g/l); after ten minutes of reaction, the hydrogel is cut up, for example into 3 mm-sided cubes.

According to this embodiment, it is possible for a bacterial propagation step to be carried out after the step of embedding a bacterial population in a cell embedding matrix, said propagation step comprising the steps of:

-   -   supplying a culture medium which allows the bacterial population         to multiply, said culture medium comprising the bacterial         population embedded in a cell embedding matrix according to a         ratio of volume of embedding matrix and of bacteria embedded         therein to the total volume of between 5% and 74%,     -   adding ferrous ions at an initial concentration of between 2 and         50 g/l,     -   replacing the culture medium with a fresh culture medium when         the oxidation of the ferrous iron to ferric iron is total, and         adding ferrous ions so as to obtain the initial concentration of         ferrous ions,     -   stopping the propagation step when the time required to oxidize         all of the ferrous iron to ferric iron within the culture medium         no longer decreases.

Preferentially, this embodiment can comprise the steps of:

-   -   adjusting the pH of the propagation medium to between 0.5 and         3.0 by adding strong acid, preferentially a sulfuric acid, and         even more preferentially to between 1.5 and 2.0,     -   oxygenating the medium using compressed air at an aeration rate         of between 0.1 vvm and 2.0 vvm (volume of air injected into the         reaction medium per volume of reaction medium and per minute),         the air being optionally enriched with 0.1% to 10% of CO₂. The         diffusion of the air within the reactor can be carried out by an         element which provides a volumetric oxygen transfer coefficient         (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about         0.030 s⁻¹ to 0.050 s⁻¹. Such an element can be a toroidal         diffuser, or a fine bubble diffuser made of ceramic or of         elastomers of EPDM type.

The culture medium may, for example be a conventional medium of Lundgren-Silverman 9K type, the pH of which is between 0.5 and 3.0, in the presence of an FeSO₄.7H₂O concentration of between 2 g/l and 50 g/l.

This propagation step is carried out until a stability of the time required to oxidize all of the iron included in the culture medium is reached and until this time no longer decreases, indicating maximum colonization of the matrix. The concentrations of the various forms of the iron present in the culture medium can be monitored according to the methods already previously described.

According to one particular embodiment of the invention, a propagation step consisting of a step of promoting the growth of the bacteria in the preculture intended to provide the inoculum which will be embedded in the embedding matrix is performed. This step of propagating a bacterial population is carried out before the step of embedding the bacterial population in a matrix, said propagating step comprising the step of:

supplying a propagating medium comprising an iron(II) concentration of between 2 and 50 g/l, the pH of said propagating medium being between 0.5 and 3.0,

supplying a bacterial inoculum representing from 2% to 10% (v/v) of the culture.

The propagating medium is generally a medium of 9K type, the pH of which is adjusted to between 0.5 and 2.0. Generally, the medium is acidified using a concentrated solution of H₂SO₄. The iron can be supplied in the form of FeSO₄.7H₂O. Preferentially, the conditions for growth of the bacterial population for the purpose of preparing an inoculum and which is intended to be embedded in a matrix are, in a bioreactor of stirred tank reactor type, a flow rate for aeration of the medium of between 0.1 and 2.0 vvm (volume of air per volume of culture per minute), the air being optionally enriched with CO₂ (from 0.1% to 10%), stirring ensuring suspension of the materials and a volumetric oxygen transfer coefficient (Kla) between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹, and the temperature is between 15° C. and 45° C. in the case of the use of mesophilic bacteria, 4° C. to 15° C. in the case of psychrotolerant bacteria, and up to 70° C. and more in the case of moderately thermophilic or hyperthermophilic bacteria. The diffusion of the air within the reactor can be carried out by an element which ensures a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹. Such an element can be a toroidal diffuser or a fine bubble diffuser made of ceramic or of elastomers of EPDM type. The bacterial growth and activity are monitored by microscopic bacterial counting and by means of a redox potential probe, said potential increasing throughout the growth of the bacterium and the oxidation of the ferrous iron to ferric iron. The bacterial growth step can be stopped when the growth medium contains approximately 10⁸ cells/ml.

According to one preferential embodiment, the process according to the invention is characterized in that the first reaction space comprises a first bacterial population (B1) embedded in a first cell embedding matrix, said first bacterial population having, prior to its use in step a) of the process and when iron is present within the material, undergone an acclimatization by culturing said first bacterial population in an acclimatization medium, said acclimatization medium comprising the material, said acclimatization medium being replaced with a fresh acclimatization medium when the redox potential of the acclimatization medium reaches a value greater than 350 mV, preferentially greater than 400 mV, and even more preferentially greater than 450 mV, said fresh acclimatization medium being supplemented at each replacement with an increasing concentration of material, said increasing concentration evolving in steps of 2 g/l to 20 g/l of material, until the concentration of the material in the medium is between 2 g/l and 200 g/l, more preferentially between 50 g/l and 150 g/l, and even more preferentially the concentration is equal to 70 g/l and 80 g/l.

The redox potential of the medium can be measured using two half-cells, or any electrode known to those skilled in the art which makes it possible to experimentally measure an oxidation-reduction potential. The threshold of 550 mV, preferentially 600 mV, and even more preferentially 650 mV, corresponds to the predominant presence, under the operating conditions, of iron in the oxidation state Fe(III).

Since the conditioning of the bacterial populations may comprise use of the material that will be bioleached, the conditioning may be present in the process prior to the step of supplying a ferrous ion. This step is carried out after the embedding of the bacterial population in a matrix. This step makes it possible to obtain a bacterial population of which the metabolism is adapted to the use thereof in one of the embodiments of the process, making it more polyvalent and robust with respect to any toxic effects of elements dissolved from the material and more effective through the adaptation of the embedded bacterial population by promoting within the matrix the development of one or more bacterial genus or genera or of one or more bacterial species included in the initial mixed culture (or initial bacterial consortium) and which exhibit(s) greater affinity and effectiveness with respect to the reaction exploited in the step under consideration.

According to one preferential embodiment, the process according to the invention is characterized in that the second reaction space comprises a second bacterial population (B2) embedded in a second cell embedding matrix, said second bacterial population having, prior to its use in step b) of the process and when an iron is present within the material, undergone an acclimatization by culturing said second bacterial population embedded in a cell embedding matrix in a second acclimatization medium, said second acclimatization medium consisting of the various successive acclimatization media resulting from the step of acclimatization of the first bacterial system from which the material was removed, the concentration of ferrous iron being maintained at a concentration of between 2 g/l and 50 g/l in said acclimatization medium, said acclimatization medium being optionally supplemented by the addition of ferrous iron.

According to one preferential embodiment, the process according to the invention is characterized in that the third reaction space comprises a third bacterial population (B3) embedded in a third cell embedding matrix, said third bacterial population having, prior to its use in step c) of the process, undergone an acclimatization by culturing said third bacterial population embedded in a third cell embedding matrix in a third acclimatization medium, said third acclimatization medium consisting of the various successive media resulting from the step of acclimatization of the second bacterial system and being supplemented with a concentration increasing in steps of 2 g/l to 20 g/l of treated material, said treated material consisting of the first material after acclimatization of the first bacterial population when the material contains iron, or the material resulting from the step of extracting substances inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity, when the material does not contain iron, or the acidified material when the material contains neither an iron nor a substance inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity, up to a concentration equivalent to 20 g/l to 200 g/l, preferentially from 50 g/l to 150 g/l, and even more preferentially from 70 g/l to 80 g/l of material in the reaction medium.

According to one preferential embodiment, the process is characterized in that a ratio of the volume of the bacterial population embedded in a cell embedding matrix to a total volume within an acclimatization space is between 5% and 74%.

Preferentially, the method comprises a step of extracting the at least one metal carried out using the third reaction medium at the end of the bioleaching step.

This extracting step consists, in addition to the bioleaching steps, in recovering the metals of interest solubilized in ionic form. This step can use chemical, physical or electrochemical (hydrometallurgical, biohydrometallurgical) processes. The technique(s) used will consist in separating the metals of interest from the reaction medium selectively when this proves to be possible, for example by redox reaction, cementation, adsorption on active carbon, precipitation, complexation, electroplating, or separation on ion exchange resins. By way of examples, a step of cementation of copper with iron metal (filings, powder, etc.), of cementation of gold with metal zinc, or of precipitation of copper with calcium carbonate can be envisioned. A biohydrometallurgical technique can consist in precipitating the solubilized metals by adjusting the pH and adding sulfides resulting from a process exploiting the sulfate-reducing activity of certain microorganisms.

Preferentially, the bacterial populations (B1, B2, B3) comprise acidophilic bacteria in pure cultures or in mixed cultures having at least one characteristic among each of the following groups selected from the groups comprising heterotrophic, mixotropic, autotrophic, chemoautotrophic or chemolithoautotrophic bacteria which oxidize iron and/or sulfur and/or reduced forms of sulfur, psychrophilic, mesophilic, moderately thermophilic, hyperthermophilic and acidophilic bacteria, said bacteria having a growth and activity pH of between 0.5 and 3.0, and more preferentially the group comprising the bacteria Acidiferrobacter thiooxydans, Acidithiobacillus ferrooxidans, Leptospirillum ferroxidans, Leptospirillum ferriphilum, Acidimicrobium ferrooxidans Sulfobacillus the rmosulfidooxidans, Acidithiobacillus caldus, Acidianus brierley, Sulfobacillus acidophilus, Actinobacterium sp., Acidocaldus organivorans and Alicyclobacillus ferroplasma.

Preferentially, during the step of bioleaching at least one metal, a step of acidolysis of a noble metal present in the material is present. Alternatively, the material, after the step of bioleaching a metal, undergoes a step of recovering the noble metals.

The acidolysis step is carried out by the first bacterial population and the third bacterial population present respectively within the first reaction space and the third reaction space. The step of recovering a noble metal present within the material after the bioleaching step can be carried out by cyanidation or biocyanidation when the noble metal is gold, silver, palladium or platinum.

According to one preferential embodiment, the process according to the invention may comprise the following steps, said steps being carried out after the step of bioleaching at least one metal:

-   -   precipitating, within a reaction medium, the ferric ion to give         iron(III) hydroxide,     -   separating the iron(III) hydroxide from the reaction medium,     -   solubilizing the iron(III) hydroxide,     -   using the solubilized iron hydroxide during a step (of the         process) of bioleaching a metal of interest.

The step of precipitating the ferric ion can be carried out by adding a sodium hydroxide concomitant with a rise in pH in order to obtain a pH of between 3.50 and 4.05, and preferentially a pH equal to 3.75. This step makes it possible to selectively precipitate an iron(III) hydroxide without precipitating the at least one metal of interest. The separating step can be carried out by any common separation method for separating an iron, such as decanting, followed by phase separation, by filtration and/or by centrifugation. The solubilization of the iron(III) hydroxide can be carried out by acidification in an aqueous solution or in a medium of Lundgren-Silverman 9K type by adding sulfuric acid, the pH of the solution being between 0.5 and 3.0, and more preferentially between 1.5 and 2.0.

EXAMPLES Example 1 Recovery of Copper from Brake Pad Dust

The waste treated is brake pad dust; this pulverulent material results from the automobile industry, and in particular from the steps of polishing, reaming and finishing treatment of the parts in question. The composition of the materials can vary according to the evolution of motor vehicle braking technologies and the batches of materials under consideration. The residue exploited in the context of this example has the following elemental composition as % by weight: Cu 15%, Fe 10%, Zn 2%, Mg 4.5%, Al 5%, but also Sb 0.4%, As 0.2%, Pb 0.5%, and other elements in trace amounts or in insignificant amount. The process is applicable to all types of waste described in this patent.

The bacteria exploited in this example are part of the class of iron(II)-oxidizing acidophilic bacteria: Acidiferrobacter thiooxydans (DSM 2392), Acidithiobacillus ferrooxidans (DSM 11477), Leptospirillum ferrooxidans (DSM 2391). These acidophilic bacteria have an optimal pH of less than 2.0. The growth medium ideally used is the 9K medium: (NH₄)₂SO₄ 4 g/l, MgSO₄.7H₂O 0.5 g/l, K₂HPO₄ 0.5 g/l, KCl 0.1 g/l, CaCl₂.2H₂O 0.13 g/l, Ca(NO₃)₂ 0.01 g/l, adjusted to pH 1.75 using a concentrated solution of H₂SO₄. The medium is supplemented with a source of Fe(II), in an amount of 50 g/l of FeSO₄.7H₂O. The bacterial growth is monitored by counting under a phase contrast microscope and the microbial activity is evaluated using a redox potential probe; the value of this parameter increases throughout the bacterial culture (from ±350 mV to ±550 mV). The volume of this culture, which will be the subject of a step of embedding in a colonized matrix mass, is 6.5 liters at a cell density of 10⁸ cells/ml. The preculture is centrifuged with a minimum acceleration of 12000 g. The suspension with a high concentration of cells, or pellet, is then diluted, for the purpose of rinsing and removing the iron, in 100 ml of iron-free 9K medium at pH 1.75; the suspension is again concentrated by centrifugation at a minimum of 12000 g and is resuspended in a volume of 50 ml of iron-free 9K medium. The embedding matrix is of semi-synthetic type: polyvinyl alcohol (PVA) (Roth MW: 72000 g/mol, degree of hydrolysis of 99.9%), and sodium alginate are used as embedding matrix component at respective final concentrations of 9% and 1% in order to encapsulate the bacteria. The PVA-alginate mixture is dissolved and sterilized by heating for 20 min at 121° C. After cooling of the embedding solution, the concentrated suspension of bacteria is mixed and homogenized with the PVA/alginate matrix in a final volume of 1 liter. The mixture is then extruded dropwise by gravitation or under pressure, which leads to the formation of beads/spheres 3-4 mm in diameter in contact with the polymerization solution consisting of 50 to 60 g/l of H₃BO₃ and 10 to 20 g/l of CaCl₂.2H₂O in a volume of 10 liters; the polymerization of the beads is prolonged with stirring at 50 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)) for 1 h. The embedding beads are subsequently rinsed with distilled water, then incubated in one liter of 9K medium, at pH 1.75, in the presence of 50 g/l of FeSO₄.7H₂O, with stirring at 150 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)) and aeration of 1 vvm, the air being optionally enriched with CO₂ (from 0.1% to 10%). The diffusion of the air within the reactor is carried out by an element (toroidal diffuser, or fine bubble diffuser made of ceramic or of elastomers of EPDM type (for example), etc.) which ensures a volumetric oxygen transfer coefficient (Kla) of between 0.010 s⁻¹ and 1 s⁻¹, preferentially of about 0.030 s⁻¹ to 0.050 s⁻¹. Following the oxidation of all the iron present in the medium (minimum 200 h in the first batch), this step is repeated two to three times until reproducibility of the iron(II) oxidation time during each propagation batch is achieved (40 to 60 h).

Physicochemical Separation/Modification of the Components Inhibiting the Bioleaching Biological Activity (Step Preliminary to the Bioleaching Process: Preconditioning Step, i.e. Step of Extracting the Inhibitory Substances and Acidifying the Material):

The separation is carried out by means of a process of suspending the brake dust residues (BDRs); this preconditioning step is carried out in 9K medium in the absence of ferrous iron and of ferric iron at pH 1.75 or in an aqueous solution of which the pH is adjusted to 1.75 with concentrated sulfuric acid, with stirring at 100 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)). The pH is ideally maintained at 1.75, ideally by automated addition of sulfuric acid. The brake dust is suspended by adding a surfactant such as Tween 80 (0.1% v/v) in order to ensure dispersion and homogeneity of the suspension and because of the hydrophobic nature of the dust. The preconditioning of the waste is carried out at ambient temperature (20° C.). The weight of waste preconditioned represents 10% of the volume of preconditioning solution. The contact time between the acid solution and the waste is ideally one hour in order to avoid dissolution of the metals of interest by acidolysis while performing longer reaction times. The sedimented solid fraction is physically separated from the preconditioning solution and from the less dense solid phase which is floating or in suspension, which constitute two forms of waste in the process. The heavy fraction is retained for the purpose of subsequent biological extraction of the metals.

Leaching of the Iron Intrinsic to the Waste (Step 1: Step of Supplying a Ferrous Ion):

This step is justified in the case of brake dust having an intrinsic iron content of ±10%, which may be the subject of mobilization or of bioleaching and thus dispenses with the process of adding iron in FeSO₄ form. In order to acclimatize the bacteria to this step of leaching the iron intrinsic to brake dust, the ratio of bead weight of 20%, i.e. 200 g of beads, nonacclimatized/liter of iron-free 9K medium is used, at pH 1.75; aeration 1 vvm (the air being optionally enriched with CO₂ (from 0.1% to 10%) for the anabolic needs of the bacteria) and stirring of 150 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)). Within the bioreactor of stirred tank reactor type or the gas lift reactor (such as air lift reactor), the beads are maintained in a synthetic cylindrical structure made of polypropylene having an open mesh of 1 mm, allowing transfers of matter, also making it possible to protect the matrix against the abrasive effects of the stirring and the waste during treatment. The acclimatization begins by adding 10 g/l of preconditioned brake dust to the bioreactor, allowing indirect action of the bacteria on the waste and acclimatization of said bacteria. Techniques for assaying by colorimetry are used in order to measure the concentration of Fe(II) leached (ortho-phenanthroline and hydroxylamine). When, in this example, all the iron is in solution (by comparison with the elemental analysis of the waste and by observation of a maximum and stable leached concentration), it is important to break, by simple removal, the contact between the waste and the bacteria in order to avoid leaching of the non-ferrous metals. The step is subsequently repeated in order to gradually increase the concentrations of waste in the reactor: 10 g/l, 25 g/l, 50 g/l up to 75 g/l of waste. During this acclimatization step, the encapsulated bacteria have access to no source of iron other than that present in the waste; this enables an adaptation of the metabolism and of the consortium within the matrix. It is considered that the acclimatization is optimal when the time required to leach all the iron intrinsic to the dust is constant. The solid waste resulting from this step makes it possible to acclimatize the bacterial populations used in step 3, the residual solutions having increasing concentrations of iron(II) make it possible to acclimatize the bacterial populations used in step 2. When the acidophilic bacteria are acclimatized to 75 g/l of brake dust, a semi-continuous system is started up allowing bioleaching of the intrinsic Fe(0). The reactor and the parameters used during the steps of the bioleaching process are identical to those used during the acclimatization. Throughout the process, the pH is maintained at 1.75, by automated addition of sulfuric acid produced here by a related culture of non-embedded Acidithiobacillus thiooxidans on 9K medium supplemented with sulfur in a proportion of 10 g/l (temperature: 30° C., aeration: 1 vvm). The action of the acclimatized embedded bacteria on the waste allows the leaching of 100% of the iron remaining after preconditioning of the brake dust over the course of 24 h. The solution of Fe(II) will then be directly brought into contact with the microorganisms allowing the oxidation of the Fe(II) to Fe(III) (step 2).

Oxidation of Fe(II) to Fe(III) (Step 2 of the Invention):

In the same way as in the previous steps, the microorganisms used in this second step have a metabolism which becomes specific within a consortium which is naturally adapted, in this case to the exclusive oxidation of the Fe(II) resulting from the iron leaching step (step 1 of the invention). This acclimatization step creates contact with all the elements derived from the brake dust treated in step 1 that can thus harm the activity, the viability and the growth of the bacteria. As in the previous step, 200 g of non-acclimatized beads, with optimal stirring and aeration, are brought into contact with one liter of reaction medium, i.e. the solution of Fe(II) resulting from step 1. The bacteria are brought into contact in successive cultures with the solutions of Fe(II) originating from 10 g/l, 25 g/l, 50 g/l then 75 g/l of brake dust treated in step 1. The parameters are an aeration at 1 vvm (the air being optionally enriched with CO₂ (from 0.1% to 10%)), a temperature of 30° C., stirring of 150 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)) and pH maintained close to 1.75 (by automated addition of sulfuric acid diluted in 9K medium, produced by Acidithiobacillus thiooxidans). Tools for measuring redox potential and for assaying ferrous/ferric iron are also used here in order to monitor the oxidation of Fe(II) to Fe(III) by the bacteria. After acclimatization of the embedded bacteria in step 2, the beads are brought into contact, in semi-continuous mode, with the solutions of Fe(II) (>10 g/l Fe(II)) resulting from the treatment of 75 g/l of brake dust. The reactor used and the parameters are identical to those used during the acclimatization. The use of techniques for measuring the redox potential and the concentration of Fe(II) makes it possible to determine the end of each Fe(II) oxidation batch, i.e. a change in the potential of 350 mV to 600 mV. When the redox potential is >550 mV, in this case in 40 to 60 h, the solution of Fe(III) resulting from the total oxidation of the Fe(II) can be brought into contact with the acclimatized bacterial populations of step 3. The pH is ideally maintained around 1.75 in order to minimize the precipitation of the Fe(II) or Fe(III), in the form of jarosite, for example, while promoting the growth and the activity of the bacterial populations; this pH value can be adjusted according to the microorganisms, the mode of encapsulation and the waste/residues treated (in particular with lower pH values from a viewpoint of the studies by Jin-yan et al., Study of formation of jarosite mediated by thiobacillus ferrooxidans in 9K medium. Proc. Earth and Planetary Sci. 1(2009)706-712).

Leaching of the Non-Ferrous Metals (Step 3: Bioleaching the Metal of Interest):

According to the same course as the previous steps of acclimatization to brake dust, 200 g of (non-acclimatized) beads per liter of reaction medium, in this case a solution of Fe(III) resulting from step 2, are used. The embedded bacteria are brought into contact with the waste resulting from step 1 of the process, according to increasing concentrations of 10 g/l, 25 g/l, 50 g/l then 75 g/l of initial treated waste. The beads are maintained in a synthetic cylindrical structure of polypropylene type having an open mesh of 1 mm. The parameters are ideally: oxygenation 1 vvm, a temperature of 30° C., stirring at 150 rpm (Rushton six-blade turbine; ratio between height of liquid Z(m) and diameter of the reservoir T (m): Z/T=1; turbine diameter D(m)=T/3; turbine position relative to the bottom of the reservoir: Z/3 (m)) and pH maintained close to 1.75. Assaying of the copper in solution makes it possible to monitor the evolution of the acclimatization to leaching of the metals. At each new batch of higher concentration, a minimum of 80% of the copper initially contained in the residues must be leached. By virtue of the acclimatization, the embedded bacteria are capable of withstanding high concentrations of solubilized metals present in the reaction medium (heavy metals), throughout the bioleaching process. They are used in the semi-continuous process, enabling bioleaching of most of the metals making up the 75 g/l of brake dust in a minimum time of 40 to 60 h, with a leaching yield of 80% to 100%. The beads are also maintained in a synthetic cylindrical structure of polypropylene type having an open mesh of 1 mm. The reactor used and the parameters are identical to those used during the acclimatization: an oxygenation of 1 vvm, a temperature of 30° C., stirring at 150 rpm (same conditions as previously) and pH maintained close to 1.75. At each end of batch, the redox potential is >550 mV and the iron is exclusively in Fe(III) form; the next batch can be started by cyclic repetition of the operations. The process set out in this example can be applied on a larger scale by adhering to all the working parameters and by scaling-up the bioreactors, in particular by transposing the volumetric oxygen transfer coefficient (Kla)

Example 2 Recovery of Non-Ferrous Metals, and of Noble Metals of Interest, by Bioleaching Ground Mobile Telephone Materials

This second example consists of the use of embedded acidophilic bacteria in order to leach heavy metals such as copper combined, in the structure of the waste, with noble metals such as gold. This makes it possible to bioleach the copper but also a fraction of the gold by acidolysis. This example is illustrated by the treatment of a material resulting from the grinding of mobile telephones (grinding of whole mobile telephones, of random brands and models, to an average particle size of 1 mm), with an initial concentration of Au of 221 mg/kg and of Cu of 69 g/kg. For this type of waste, it is not necessary to perform a separation or a preconditioning; however, an acidification of the waste is carried out. The addition of iron in FeSO₄.7H₂O form is essential given the absence of iron intrinsic to the waste. The process will therefore be carried out in two steps: oxidation of the ferrous ions (Fe(II)) (step 2 of the invention), then bioleaching of the heavy metals and of a fraction of the precious metals (step 3 of the invention). The pH is maintained close to 1.75 by addition of sulfuric acid and automated regulation throughout the process, in order to avoid iron leaks through jarosite precipitate formation.

The waste (in this case 100 g) is acidified in one liter of 9K medium at pH 1.75, by addition of H₂SO₄ until a pH stable at this value is obtained. The waste (100 g/l) is then recovered by filtration (Macherey Nagel 616 ¼ filter) or decanting (Imhoff cone) and phase separation. The cell embedding protocol is the same as the one used during the bioleaching of the metals of interest present in the brake dust in example 1: 6.5 liters of preculture are centrifuged at an acceleration of more than 12000 g, and the recovered pellet is rinsed (at least twice) in 100 ml of iron-free 9K medium, pH 1.75. The pellet is then homogenized with the dissolved elements of the embedding matrix (9% PVA/1% sodium alginate final concentration, sterilized for 20 min at 120° C.), by stirring, and the mixture is then extruded by gravitation dropwise or in interrupted-net form, thus forming beads/spheres which allow bacterial encapsulation, in a polymerization solution composed of 50 g/l H₃BO₃ and 20 g/l CaCl₂.2H₂O with stirring for 1 h. The beads are subsequently placed in 9K medium, in a proportion of 200 g of beads per liter, with 50 g/l of FeSO₄.7H₂O, pH 1.75, 30° C., with stirring in order to allow the bacteria to propagate in the matrix. Monitoring of the redox potential (from 350 mV to >550 mV) makes it possible to determine the oxidation state of the iron (10 g/l) in solution. After having carried out the propagation phase (four successive batches of oxidation of the iron by the bacteria inoculated in the matrix), i.e. having obtained a constant iron oxidation time from batch to batch of 50 h, 200 g of beads (for one liter of 9K medium, 50 g/FeSO₄.7H₂O, pH 1.75, redox potential 350 mV) will be exclusively used to oxidize the iron (step 2 of the invention). The beads are kept immersed in the medium by virtue of a synthetic structure and the parameters are ideally: oxygenation 1 vvm, a temperature of 30° C., stirring at 150 rpm (same conditions as previously). Assaying by colorimetry with ortho-phenanthroline and hydroxylamine, and monitoring of the redox potential (of >550 mV), make it possible to determine the time starting from which the oxidation of the Fe(II) to Fe(III) is total, i.e., in this example, 100% of the Fe(II) oxidized to Fe(III) in 50 h. Ideally, the pH is maintained at 1.75 by adding sulfuric acid of biological or chemical origin. The solution of Fe(III) will make it possible to initiate the bioleaching of the metals of interest in the next step.

During the second part of this process (step 3 of the invention), 200 g of colonized PVA/alginate beads (for one liter of reaction medium) having also followed the propagation phase will be acclimatized for bioleaching of the heavy metals (copper); the beads are kept immersed in a synthetic cylindrical structure of polypropylene type having an open mesh of 1 mm. The ideal parameters are: oxygenation 1 vvm, a temperature of 30° C., stirring at 150 rpm (same conditions as previously) and pH maintained close to 1.75. The solution of Fe(III) resulting from the previous reactor is used here as reaction medium (10 g/l Fe(III)). For the acclimatization, the beads are brought into contact with a first concentration of acidified waste, 10 g/l, thus causing a drop in the redox potential of the reaction solution by reduction of the Fe(III) to Fe(II) by reactions with the metals contained in the ground telephone materials (from 600 mV to 350 mV). Monitoring the redox potential makes it possible to determine the evolution of the leaching of the metals; when the potential has come back up to 550 mV, the reaction is ended, the metals are leached. The solid waste and the bioleaching solution are separated by decanting (Imhoff cone) and phase separation or by filtration, and then the oxidation step is repeated with one liter of fresh 9K medium supplemented with 50 g/l of FeSO₄.7H₂O (pH 1.75). Step 3 of the invention is then carried out with an addition of waste (20 g/l, 30 g/l, 40 g/l, etc.) to the solution of Fe(III) resulting from the previous reactor, up to the working concentration of material, i.e. 65 g/l of treated ground mobile telephone material waste.

Once the beads have been acclimatized, a semi-continuous pilot will make it possible to treat 65 g/l of telephone waste in a time of about 48 h. The results from quantification of metals that have been solubilized by this process (ICP, colorimetric assay, etc.) show that a minimum of 50% of the copper present in the waste is leached, but also 27% of the gold. In order to improve the yield of the process, after step 3, a step of precipitation of the iron (Fe(III)) by addition of 1M NaOH, in the form of Fe(OH)₃, until the pH is increased to the value of 3.75, is carried out. At this pH, an amount>95% of the iron present in the medium is precipitated, causing less than 10% of precipitation of the bioleached metals other than iron (Cu, Zn, Ni, etc.).

The redox potential or oxidation-reduction potential values set out in the description of this invention are expressed relative to an Ag/AgCI reference electrode. The absolute and conventional values relative to the normal hydrogen electrode (E^(o)) are obtained by addition to the mentioned values of the potential of the reference electrode relative to the NHE corrected with respect to the working temperature, i.e. 192 mV at 30° C. in the case of an Ag/AgCl/KClsat+AgClsat electrode.

The present invention has been described in relation to specific embodiments which have a purely illustrative value and should not be considered to be limiting. Generally, it will appear obvious to those skilled in the art that the present invention is not limited to the examples illustrated and/or described above. The use of the verbs “to comprise”, “to include”, “to contain”, or any other variant, and also conjugations thereof, cannot in any way exclude the presence of elements other than those mentioned.

The use of the indefinite article “a”, or of the definite article “the”, to introduce an element does not exclude the presence of a plurality of these elements.

The invention can also be described as follows:

A process for recovering at least one metal present within a material, said material possibly comprising iron, the process comprising a step of supplying a ferrous ion, a step of supplying a ferric ion and a step of bioleaching at least one metal present in the material via the ferric ions, each of the steps being implemented by a particular bacterial population. 

1. An indirect bioleaching process for extracting at least one metal other than iron present in a material, said process comprising the steps of: a) supplying a ferrous ion in a first reaction space, b) oxidizing the ferrous ion supplied to give a ferric ion in a second reaction space, c) bioleaching said at least one metal via said ferric ion in a third reaction space, characterized in that the step of supplying a ferrous ion is carried out, i) either by bioleaching of an iron present within the material by a first iron-oxidizing bacterial population (B1) which is embedded in a first embedding matrix and immersed in a first reaction medium present within the first reaction space, ii) or by acid leaching of an iron present within the material by adding a strong acid, preferentially sulfuric acid, to the first reaction space, iii) or by adding ferrous ions, so as to obtain a ferrous ion concentration of between 2 g/l and 50 g/l in the first reaction medium, and in that the steps of oxidizing the ferrous ion and of bioleaching said at least one metal are each carried out in a reaction space comprising a reaction medium and a bacterial population (B2, B3) comprising at least one population of iron-oxidizing acidophilic bacteria, each of the bacterial populations (B2, B3) being embedded in a cell embedding matrix immersed in each of the reaction media of each reaction space, it being possible for the reaction media, the bacterial populations and the materials present in the various reaction spaces to be transferred between these various reaction spaces, and in that an iterative oxidation of a ferrous ion, resulting from the bioleaching of said at least one metal, to give a ferric ion is carried out by the bacterial population (B3) present in the third reaction space, and in that the reaction spaces are different and the bacterial populations (B1, B2, B3) are not identical.
 2. The process as claimed in claim 1, characterized in that said step of bioleaching said at least one metal is reiterated when the amount of said at least one metal in the material after a bioleaching step is greater than 20% of the initial amount of said at least one metal within the material, and more preferentially greater than 50%.
 3. The process as claimed in claim 1, characterized in that the process also comprises at least one of the following steps: extraction of at least one substance present in said material and inhibiting a bioleaching bacterial activity and/or an iron-oxidizing bacterial activity, said step being carried out before the step of supplying a ferrous ion, acidification of the material by supplying an acid solution such that the material is in a solution of which the pH is stabilized between 0.5 and 3.0.
 4. The process as claimed in claim 1, characterized in that a step of embedding a bacterial population in a cell embedding matrix is present and in that the embedding step comprises the steps of: supplying a liquid embedding medium, supplying a cell embedding matrix compound in the embedding medium according to a weight-to-volume ratio between the embedding matrix compound and the embedding medium of between 5 g/l and 400 g/l, supplying an inoculum of the bacterial population in the embedding medium, forming a cell embedding matrix comprising the bacterial population.
 5. The process as claimed in claim 4, characterized in that the cell embedding matrix compound is selected from the group of water-soluble natural or synthetic polymers which can form hydrogels by ionotropic, photochemical or thermal gelling or by crosslinking or polymerization, said group comprising alginate, agar, gerlite (anionic heteropolysaccharide), chitosan, kappa-carrageenan, polyacrylamide, polyacrylamide-hydrazide, co-poly(N-isopropylacrylamide/acrylamide), polyethylene glycol, methacrylates based on monomers of methylacrylamide, hydroxy ethyl methacrylate or methyl methacrylate, epoxy resins, photo-crosslinkable resins, diethylene glycol ester, polyvinylpyrrolidone, silicone, polyvinyl alcohol, polyethylene glycol monomethacrylates or dimethacrylates or diacrylates, polyurethane hydrogel, hydroxyethyl methacrylate, photo-crosslinkable resins or a mixture of several of these compounds.
 6. The process as claimed in claim 4, characterized in that a bacterial propagation step is carried out after the step of embedding a bacterial population in a cell embedding matrix, said propagation step comprising the steps of: supplying a culture medium which allows the bacterial population to multiply, said culture medium comprising the bacterial population embedded in a cell embedding matrix according to a ratio of volume of embedding matrix and of bacteria embedded therein to the total volume of between 5% and 74%, adding ferrous ions at an initial concentration of between 2 and 50 g/l, replacing the culture medium with a fresh culture medium when the oxidation of the ferrous iron to ferric iron is total, and adding ferrous ions so as to obtain the initial concentration of ferrous ions, stopping the propagation step when the time required to oxidize all of the ferrous iron to ferric iron within the culture medium no longer decreases.
 7. The process as claimed in claim 1, characterized in that the first reaction space comprises a first bacterial population (B1) embedded in a first cell embedding matrix, said first bacterial population having, prior to its use in step a) of the process and when iron is present within the material, undergone an acclimatization by culturing said first bacterial population in an acclimatization medium, said acclimatization medium comprising the material, said acclimatization medium being replaced with a fresh acclimatization medium when the redox potential of the acclimatization medium reaches a value greater than 350 mV, preferentially greater than 400 mV, and even more preferentially greater than 450 mV, said fresh acclimatization medium being supplemented at each replacement with an increasing concentration of material, said increasing concentration evolving in steps of 2 g/l to 20 g/l of material, until the concentration of the material in the medium is between 2 g/l and 200 g/l, more preferentially between 50 g/l and 150 g/l, and even more preferentially the concentration is equal to 70 g/l and 80 g/l.
 8. The process as claimed in claim 7, characterized in that the second reaction space comprises a second bacterial population (B2) embedded in a second cell embedding matrix, said second bacterial population having, prior to its use in step b) of the process and when an iron is present within the material, undergone an acclimatization by culturing said second bacterial population embedded in a cell embedding matrix in a second acclimatization medium, said second acclimatization medium consisting of the various successive acclimatization media resulting from the step of acclimatization of the first bacterial system from which the material was removed, the concentration of ferrous iron being maintained at a concentration of between 2 g/l and 50 g/l in said acclimatization medium, said acclimatization medium being optionally supplemented by the addition of ferrous iron.
 9. The process as claimed in claim 8, characterized in that the third reaction space comprises a third bacterial population (B3) embedded in a third cell embedding matrix, said third bacterial population having, prior to its use in step c) of the process, undergone an acclimatization by culturing said third bacterial population embedded in a third cell embedding matrix in a third acclimatization medium, said third acclimatization medium consisting of the various successive media resulting from the step of acclimatization of the second bacterial system and supplemented with a concentration increasing in steps of 2 g/l to 20 g/l of treated material, said treated material consisting of the first material after acclimatization of the first bacterial population when the material contains iron, or the material resulting from the step of extracting substances inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity, when the material does not contain iron, or the acidified material when the material contains neither an iron nor a substance inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity, up to a concentration equivalent to 20 g/l to 200 g/l, preferentially from 50 g/l to 150 g/l, and even more preferentially from 70 g/l to 80 g/l of material in the reaction medium.
 10. The process as claimed in any one of claims 7 to 9, characterized in that a ratio of a volume of the bacterial population embedded in a cell embedding matrix to a total volume within an acclimatization space is between 5% and 74%.
 11. The process as claimed in claim 1, characterized in that step a) of supplying ferrous ions is carried out with a ratio of the volume of the first bacterial population (B1) embedded in a first cell embedding matrix to the volume of the first reaction space of between 5% and 74%, when an iron is present in the material, in that the pH of said first reaction medium is between 0.5 and 3.0, and in that the concentration of material within the first reaction space is, when said material contains an iron, between 20 g/l and 200 g/l, preferentially between 50 g/l and 150 g/l, and even more preferentially between 70 g/l and 80 g/l of material in the reaction medium.
 12. The process as claimed in claim 1, characterized in that step b) of oxidizing the ferrous ions to ferric ions is carried out with a ratio of the volume of the second bacterial population (B2) embedded in a second cell embedding matrix to the volume of the second reaction space of between 5% and 74%, said second reaction medium consisting of the first reaction medium after step a) of supplying a ferrous ion, and in that the concentration of ferrous ions is adjusted to between 2 g/l and 50 g/l, and in that the pH of the reaction medium is less than 3.0.
 13. The process as claimed in claim 1, characterized in that step c) of bioleaching said at least one metal is carried out with a ratio of the volume of the third bacterial population (B3) embedded in a third cell embedding matrix to the volume of the third reaction space of between 5% and 74%, said third reaction medium consisting of the second reaction medium after step b) of oxidizing the ferrous iron to ferric iron, a pH of the reaction medium being between 0.5 and 3.0, and in that the concentration of material is between 20 and 200 g/l, preferentially between 50 and 150 g/l, and even more preferentially between 70 g/l and 80 g/l of material in the reaction medium, said material consisting of the material after leaching of the iron by the first bacterial population (B1) when the material contains iron, and/or the material resulting from the step of acidification and/or extraction of substances inhibiting a bioleaching bacterial activity and/or an iron-oxidizing activity.
 14. The process as claimed in claim 1, characterized in that the bacterial populations (B1, B2, B3) comprise acidophilic bacteria in pure cultures or in mixed cultures having at least one characteristic among each of the following groups selected from the groups comprising heterotrophic, mixotrophic, autotrophic, chemoautotrophic or chemolithoautotrophic bacteria which oxidize iron and/or sulfur and/or reduced forms of sulfur, psychrophilic, mesophilic, moderately thermophilic, hyperthermophilic and acidophilic bacteria, said bacteria having a growth and activity pH of between 0.5 and 3.0, and more preferentially the group comprising the bacteria Acidiferrobacter thiooxydans, Acidithiobacillus ferrooxidans, Leptospirillum ferroxidans, Leptospirillum ferriphilum, Acidimicrobium ferrooxidans Sulfobacillus thermosulfidooxidans, Acidithiobacillus caldus, Acidianus brierley, Sulfobacillus acidophilus, Actinobacterium sp., Acidocaldus organivorans and Alicyclobacillus ferroplasma.
 15. The process as claimed in claim 1, characterized in that the material is selected from the group of industrial waste, by-products or residues comprising non-ferrous metals and/or noble metals, said material being composed of at least one metal optionally in combination with iron, said metal being selected from the group comprising copper, zinc, nickel, tin, aluminum, gold, silver, platinum, rhodium, ruthenium, iridium, osmium, palladium, titanium, cobalt, vanadium, molybdenum, tungsten, beryllium, bismuth, cerium, cadmium, niobium, technetium, indium, gallium, germanium, lithium, selenium, tantalum, tellurium, arsenic, antimony, bismuth, lead, and mercury, or a combination of these metals.
 16. The process as claimed in claim 1, characterized in that the material is in the form of powder and has a particle size of less than 1 mm, preferentially less than 0.5 mm. 